1. Field of the Invention
The present invention broadly relates to an upper surface blowing jet propulsion system in a STOL (short takeoff and landing) aircraft, and, more particularly, is concerned with a method and apparatus for controlling the direction of the thrust vector of the exhaust stream from the jet engine of the aircraft.
2. Description of the Prior Art
STOL aircraft are sometimes designed with an upper surface blowing jet propulsion system, such as illustrated and disclosed in U.S. Pat. No. 4,019,696. The jet engines are mounted above and forward of the longitudinal axis of the wings so that their exhaust streams travel over the upper surface of the wings and flaps, if any, forming the trailing edge of the wings. Due to the Coanda effect, defined as the tendency of a gas or fluid stream to follow a curved surface, the respective exhaust stream attaches itself to the upper surface of the associated wing and flaps, such as seen in FIG. 2 of the aforesaid patent, the flaps being in an extended state. The exhaust stream is thereby deflected toward the ground. The vector of the thrust produced by this arrangement lies in a direction opposite to the momentum of the jet exhaust stream and gains a vertical component due to the deflection. This vertical component of the thrust vector augments the normal aerodynamic lift provided by the wings and, thus, allows an upper surface blowing type aircraft to take off in relatively short distances.
During high speed (cruise) operation, the flaps of the upper surface blowing type aircraft are retracted and little or no exhaust stream attachment to the wings and retracted flaps occurs. Thus, during cruise the jet engine exhaust stream flows directly rearwardly, but still over the upper wing and flap surfaces. Consequently, the direction of the thrust vector during aircraft cruise is generally horizontal.
A horizontal thrust vector would also be advantageous as the plane accelerates down the runway approaching takeoff. If the exhaust stream was not deflected by the wing and flaps but rather was expelled in a horizontal direction, then maximum acceleration of the aircraft on the ground would result. This would allow the normal aerodynamic wing-generated lift to increase at a maximum rate. However, to then take advantage of the upper surface blowing characteristic of the aircraft for achieving short distance takeoff, the jet engine exhaust flow should be caused to attach to the upper surface of the wing and flaps at a point in the takeoff where the additional lift would yield a total lift sufficient for immediate takeoff. But most upper surface blowing type aircraft, by utilizing movable flaps to produce the curved surface to which the jet engine exhaust stream attaches, must mechanically deploy the flaps by unfolding or extending them which takes place too slowly to achieve deflection of the exhaust stream at the opportune moment. Consequently, conventional upper surface blowing designs cannot take advantage of the maximum horizontal thrust vector during takeoff.